text: /*
    Stockfish, a UCI chess playing engine derived from Glaurung 2.1
    Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
    Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
    Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
  
    Stockfish is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
  
    Stockfish is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
  
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
  
  #ifndef MOVEPICK_H_INCLUDED
  #define MOVEPICK_H_INCLUDED
  
  #include <array>
  #include <limits>
  #include <type_traits>
  
  #include "movegen.h"
  #include "position.h"
  #include "types.h"
  
  /// StatsEntry stores the stat table value. It is usually a number but could
  /// be a move or even a nested history. We use a class instead of naked value
  /// to directly call history update operator<<() on the entry so to use stats
  /// tables at caller sites as simple multi-dim arrays.
  template<typename T, int D>
  class StatsEntry {
  
    T entry;
  
  public:
    void operator=(const T& v) { entry = v; }
    T* operator&() { return &entry; }
    T* operator->() { return &entry; }
    operator const T&() const { return entry; }
  
    void operator<<(int bonus) {
      assert(abs(bonus) <= D); // Ensure range is [-D, D]
      static_assert(D <= std::numeric_limits<T>::max(), "D overflows T");
  
      entry += bonus - entry * abs(bonus) / D;
  
      assert(abs(entry) <= D);
    }
  };
  
  /// Stats is a generic N-dimensional array used to store various statistics.
  /// The first template parameter T is the base type of the array, the second
  /// template parameter D limits the range of updates in [-D, D] when we update
  /// values with the << operator, while the last parameters (Size and Sizes)
  /// encode the dimensions of the array.
  template <typename T, int D, int Size, int... Sizes>
  struct Stats : public std::array<Stats<T, D, Sizes...>, Size>
  {
    typedef Stats<T, D, Size, Sizes...> stats;
  
    void fill(const T& v) {
  
      // For standard-layout 'this' points to first struct member
      assert(std::is_standard_layout<stats>::value);
  
      typedef StatsEntry<T, D> entry;
      entry* p = reinterpret_cast<entry*>(this);
      std::fill(p, p + sizeof(*this) / sizeof(entry), v);
    }
  };
  
  template <typename T, int D, int Size>
  struct Stats<T, D, Size> : public std::array<StatsEntry<T, D>, Size> {};
  
  /// In stats table, D=0 means that the template parameter is not used
  enum StatsParams { NOT_USED = 0 };
  enum StatsType { NoCaptures, Captures };
  
  /// ButterflyHistory records how often quiet moves have been successful or
  /// unsuccessful during the current search, and is used for reduction and move
  /// ordering decisions. It uses 2 tables (one for each color) indexed by
  /// the move's from and to squares, see www.chessprogramming.org/Butterfly_Boards
  typedef Stats<int16_t, 10692, COLOR_NB, int(SQUARE_NB) * int(SQUARE_NB)> ButterflyHistory;
  
  /// CounterMoveHistory stores counter moves indexed by [piece][to] of the previous
  /// move, see www.chessprogramming.org/Countermove_Heuristic
  typedef Stats<Move, NOT_USED, PIECE_NB, SQUARE_NB> CounterMoveHistory;
  
  /// CapturePieceToHistory is addressed by a move's [piece][to][captured piece type]
  typedef Stats<int16_t, 10692, PIECE_NB, SQUARE_NB, PIECE_TYPE_NB> CapturePieceToHistory;
  
  /// PieceToHistory is like ButterflyHistory but is addressed by a move's [piece][to]
  typedef Stats<int16_t, 29952, PIECE_NB, SQUARE_NB> PieceToHistory;
  
  /// ContinuationHistory is the combined history of a given pair of moves, usually
  /// the current one given a previous one. The nested history table is based on
  /// PieceToHistory instead of ButterflyBoards.
  typedef Stats<PieceToHistory, NOT_USED, PIECE_NB, SQUARE_NB> ContinuationHistory;
  
  
  /// MovePicker class is used to pick one pseudo legal move at a time from the
  /// current position. The most important method is next_move(), which returns a
  /// new pseudo legal move each time it is called, until there are no moves left,
  /// when MOVE_NONE is returned. In order to improve the efficiency of the alpha
  /// beta algorithm, MovePicker attempts to return the moves which are most likely
  /// to get a cut-off first.
  class MovePicker {
  
    enum PickType { Next, Best };
  
  public:
    MovePicker(const MovePicker&) = delete;
    MovePicker& operator=(const MovePicker&) = delete;
    MovePicker(const Position&, Move, Value, const CapturePieceToHistory*);
    MovePicker(const Position&, Move, Depth, const ButterflyHistory*,
                                             const CapturePieceToHistory*,
                                             const PieceToHistory**,
                                             Square);
    MovePicker(const Position&, Move, Depth, const ButterflyHistory*,
                                             const CapturePieceToHistory*,
                                             const PieceToHistory**,
                                             Move,
                                             Move*);
    Move next_move(bool skipQuiets = false);
  
  private:
    template<PickType T, typename Pred> Move select(Pred);
    template<GenType> void score();
    ExtMove* begin() { return cur; }
    ExtMove* end() { return endMoves; }
  
    const Position& pos;
    const ButterflyHistory* mainHistory;
    const CapturePieceToHistory* captureHistory;
    const PieceToHistory** continuationHistory;
    Move ttMove;
    ExtMove refutations[3], *cur, *endMoves, *endBadCaptures;
    int stage;
    Square recaptureSquare;
    Value threshold;
    Depth depth;
    ExtMove moves[MAX_MOVES];
  };
  
  #endif // #ifndef MOVEPICK_H_INCLUDED
  